334 Reports and Proceedings — Geological Society of London. 



III. — June 7th, 1905.— J. E. Marr, Sc.D., F.RS., President, in 

 the Chair. The following communications were read : — 



1, " The Microscopic Structure of Minerals forming Serpentine, 

 and their Relation to its History." By Professor T. G. Bonney, 

 Sc.D., LL.D., F.R.S., V.P.G.S., and Miss C. A. Raisin, D.Sc. 



The authors, after a brief reference to investigations of serpentine 

 during the last thirty years, which still leaves some points unsettled, 

 describe the formation of that mineral from sundry ferroraagnesian 

 silicates. Having given a summary of the changes in olivine, they 

 describe moi'e fully the alteration of separate grains in the so-called 

 ' kimbei'lite ' of South Africa. Then, after referring to the serpentini- 

 zation of amphibole, as illustrated in the well-known Rauenthal rock, 

 they enter more fully into the changes, first, of the orthorhombic 

 pyroxenes ; then of the monoclinic. To illustrate the latter, they 

 describe the conversion of malacolite into serpentine in the well- 

 known ' Eozoon ' rock of Cote St. Pierre, and of some augite-bearing 

 serpentines in the Vosges. An investigation follows of the form of 

 serpentine called ' antigorite,' described by Dr. Hussak (in 1883) 

 from Sprechenstein on the Brenner Pass, and asserted to exhibit 

 a ' netting-like ' {geslriclde) structure, which is a record of the 

 nearly rectangular cleavage of the original augite. They show, by 

 study of specimens collected in the Sprechenstein district by Miss 

 Raisin, that no such connection exists, and that the netting - like 

 sti'ucture has often only a subjective existence. 



The mica-like mineral called antigorite is shown to be abundant 

 in the Pennine Alps about the head of the Vispthal, and to lead 

 to the same conclusion, suggesting, no less than the Sprechenstein 

 specimens, a relation between that mineral and pressure. After 

 a brief notice of the chemical changes in the conversion of the 

 various anhydrous silicates into serpentine, the authors embody their 

 investigations in the following conclusions : — 



(1) That both a tint and pleochroism are accidental rather than 

 essential characteristics of antigorite. 



(2) Neither are low polarization-tints characteristic, unless two 

 mica-like minerals exist, otherwise indistinguishable. 



(3) That it is doubtful whether any hard-and-fast line can be 

 drawn between antigorite and the more fibrous forms in ordinary 

 serpentine rocks. 



(4) That the most typical antigorite appears when the rock has 

 been considerably affected by pressure, but it becomes less so when 

 the latter has been very great. 



(5) That so far from the nearly rectangular cleavage of augite 

 orio-inating the ' gestrickte struktur,' it is worse preserved than any 

 other original one in the process of serpentinization. Typical anti- 

 gorite, however, apparently is rather more readily produced from 

 augite than from the other ferromagnesian silicates, but is more 

 directly a consequence of pressure than of chemical composition. 



2. " The Tarns of the Canton Ticino." By Prof. E. J. Garwood, 

 M.A., Sec. G.S. 



The lakes dealt with comprise the larger Alpine tarns which occur 



